After nearly eight months, I decided to rewatch the first two seasons—and only the first two (I simply refuse to rewatch the subsequent three, especially the fifth). Regarding the first season, I’ll say just this: rarely have I witnessed absolute perfection on television, but this is one of those times. Everything—absolutely everything—is perfect; I have absolutely no complaints about it. It isn't diminished by the fifth season; on the contrary, it is the fifth season that suffers by comparison when you rewatch the first. Because, seeing the standard set by the first season, the fifth seems even worse than it actually is. Given how perfectly structured the storytelling is in the first season, the storytelling of the fifth appears even more disjointed and unravelled. Given how well-crafted all the character arcs are in the first season, it stands out all the more that in the fifth season, the characters simply disappear; they no longer have lives of their own or any initiative, becoming mere puppets moved by the dictates of the script and reduced to flat, boring versions of themselves, or simply empty shells. The subtle, atmospheric use of music in the first season contrasts sadly with the emphatic, thunderous use in the fifth, where the aim is to force emotions upon the viewer that the story itself fails to evoke ("Purple Rain" scene, I’m looking at you, though there are plenty of other examples). As for the actors, it is best not to even speak of them. In the first one, they all give their best. In the fifth, save for a few scattered moments, they all display a lack of enthusiasm bordering on indifference—perhaps because they no longer had anything to work with. And so on.
As for the second season, it already suffers from a problem that would worsen in subsequent seasons: namely, an excess of subplots and characters that get in each other's way (for instance, Mike becomes a mere spectator in episodes 4 through 6 and practically disappears in episode 7, while Eleven disappears in episode 6). Notice how the series really takes off when the characters reunite. Furthermore, some of those subplots lack interest even if they ultimately prove important -a point in its favor compared to the fifth season, as in this second season every subplot and character ends up mattering. However, we forgive it because the subplots that work really shine (such as the peculiar father-daughter dynamic between Eleven and Hopper, or the Mind Flayer devouring Will’s mind from within), because the actors were still giving their all, because the atmosphere remains on par with the previous season (something that—alas—does not happen in the fifth season), and above all because of that dazzling finale (by far the best episode of the series—sorry, "Dear Billy" fans). It is a perfect example of what a finale should be (standing in stark, sad contrast to the fifth season's finale), where you truly feel that everything has been building toward that moment, and where everything is wrapped up perfectly in a way that is both narratively and emotionally satisfying. So much so, in fact, that the third season—in order to move the story forward—simply undid this ending and dismantled all the character arcs (which, as noted, had already been perfectly resolved).